irq: multilevel: fix `irq_parent_level_3()`

The IRQ for level 1 and above is incremented by 1 when encoded
with Zephyr's multilevel IRQ scheme, so it should be
decremented by 1.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>
Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>